The present invention is directed to an organic light emitting device having a charge carrier layer. In particular, the charge carrier layer contains a compound having molecules that have at least one electron transporting moiety and at least one hole transporting moiety.
Organic light emitting devices (OLEDS) are comprised of several organic layers in which one of the layers is comprised of an organic material that can be made to electro-luminesce by applying a voltage across the device. C. W. Tang et al., Appl. Phys. Lett. 51, 913 (1987). Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays. S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February, 1995. Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor.
A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was reported in U.S. Pat. No. 5,703,436, Forrest et al. This TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mgxe2x80x94Ag-ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mgxe2x80x94Ag-ITO electrode layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color, red or blue. This colored emission could be transmitted through the adjacently stacked transparent, independently addressable, organic layer, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers.
U.S. Pat. No. 5,703,745, Forrest et al, disclosed an integrated SOLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. U.S. Pat. No. 5,703,745, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
Such devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, OLEDs are comprised of at least two thin organic layers between an anode and a cathode. The material of one of these layers is specifically chosen based on the material""s ability to transport holes, a xe2x80x9chole transporting layerxe2x80x9d (HTL), and the material of the other layer is specifically selected according to its ability to transport electrons, an xe2x80x9celectron transporting layerxe2x80x9d (ETL). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the HTL, while the cathode injects electrons into the ETL. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. These excitons are trapped in the material which has the lowest energy. Recombination of the short-lived excitons may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism.
The materials that function as the ETL or HTL of an OLED may also serve as the medium in which exciton formation and electro-luminescent emission occur. Such OLEDs are referred to as having a xe2x80x9csingle heterostructurexe2x80x9d (SH). Alternatively, the electro-luminescent material may be present in a separate emissive layer between the HTL and the ETL in what is referred to as a xe2x80x9cdouble heterostructurexe2x80x9d (DH).
In a single heterostructure OLED, either holes are injected from the HTL into the ETL where they combine with electrons to form excitons, or electrons are injected from the ETL into the HTL where they combine with holes to form excitons. Because excitons are trapped in the material having the lowest energy gap, and commonly used ETL materials generally have smaller energy gaps than commonly used HTL materials, the emissive layer of a single heterostructure device is typically the ETL. In such an OLED, the materials used for the ETL and HTL should be chosen such that holes can be injected efficiently from the HTL into the ETL. Also, the best OLEDs are believed to have good energy level alignment between the highest occupied molecular orbital (HOMO) levels of the HTL and ETL materials.
In a double heterostructure OLED, holes are injected from the HTL and electrons are injected from the ETL into the separate emissive layer, where the holes and electrons combine to form excitons.
Various compounds have been used as HTL materials or ETL materials. HTL materials mostly consist of triaryl amines in various forms which show high hole mobilities (xcx9c10xe2x88x923 cm2 Vs). There is somewhat more variety in the ETLs used in OLEDs. Aluminum tris(8-hydroxyquinolate) (Alq3) is the most common ETL material, and others include oxidiazol, triazol, and triazine.
For example, a typical single heterostructure device may be made of ITO/TPD/Alq3/Mgxe2x80x94Ag. ITO serves as the anode, N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2biphenyl-4,4xe2x80x2diamine (TPD) serves as the HTL, Alq3 serves as the ETL, and Mgxe2x80x94Ag serves as the cathode. When a bias is applied across the device, holes are injected from ITO into TPD and migrate to the interface between the TPD and the Alq3, and electrons are injected from the Mgxe2x80x94Ag alloy into Alq3 and move to the same interface. Holes are injected from the TPD into the Alq3, where they combine with electrons to form excitons. The excitons randomly diffuse through the Alq3 layer until they recombine, preferentially via a photoemissive mechanism. The maximum distance of exciton migration in the Alq3 layer of such a device is estimated to be around 300 xc3x85 in Alq3.
Most emissive materials used in OLEDs have either low hole mobility or low electron mobility. As a result, exciton formation typically occurs very close to the interface where the charge carrier having the lower mobility is injected into the emissive layer. For example, most ETL materials have very poor hole conducting properties, such that the excitons will be preferentially formed very close to the HTL/ETL interface in a single heterostructure OLED having an emissive ETL. Because excitons are very short lived, they do not move very far before recombining. As a result, only a small volume of the ETL is used for exciton formation and recombinations. Using only a small volume of the emissive layer for exciton formation and recombination may lead to lower lifetime for the OLED. There is therefore a need for an emissive layer having a high electron mobility and a high hole mobility, such that exciton formation can occur in a reasonable volume of the layer. For example, there is a need for an ETL having a high hole mobility, so that exciton formation and light emission can occur in a reasonable volume of the ETL of a single heterostructure OLED having an emissive ETL.
It was first thought that mixing HTL and ETL materials together would decrease the spatial separation and increase the diffusion of holes into the ETL material. This idea was first demonstrated in a variety of polymer systems where both hole transporting and electron transporting moieties were blended into the polymer matrix. The most commonly used combination is the PVK/PBD system where PVK (polyvinylcarbazole) serves as hole transporter and PBD (2-(4-t-butylphenyl)-5-(4-phenyl-1-phenylene)oxidiazole) as the electron transporter. A number of different dyes have been doped into this system as emitting centers to generate colors from blue to red.
U.S. Pat. No. 5,294,870 discloses a series of aluminum(III) complexes with two 8-hydroxyquinaldine ligands and a derivative of phenolate ligand, Alqxe2x80x22(OAr). All of these species are blue emitters and are reasonable electron transporters. Devices have been fabricated with Alqxe2x80x22(OAr) sandwiched between TPD and Alq3. This configuration is necessary because of the poor electron injection from the Mgxe2x80x94Ag electrode. Nevertheless, these devices show blue-green emissions in their electro-luminescent (EL) spectra and are nearly identical to their photo-luminescent (PL) spectra. These compounds have also shown good properties as host materials for doping with other fluorescent dyes, such as perylene to give blue emission. However, the ETL materials disclosed are poor hole transporters.
At the molecular level, Tamoto et al have designed a series of new emitting materials having an oxadiazole group as electron transporter and a triphenylamine as hole transporter. Tamoto et al, Electroluminescence of 1,3,4-Oxadiazole and Triphenylamine-Containing Molecules as an Emitter in Organic Multilayer Light Emitting Diodes, Chem. Mater. 9, 1077-1085 (1997). Layers formed of these emitting materials tend to form exciplexes with hole transporters with low ionization potentials. When exciplexes are not formed, high external quantum efficiency and energy conversion efficiency are observed. However, devices consisting of these materials suffer from low luminescent lifetimes. It was also discovered that these emitting materials have more tendency to transfer electrons than holes.
The present invention is directed to an organic light emitting device (OLED) comprising a heterostructure for producing electro-luminescence. The heterostructure has a charge carrier layer that includes a compound having molecules having at least one electron transporting moiety and at least one hole transporting moiety wherein the electron transporting moiety is a 2-methyl-8-quinolinolato ligand coordinated with a Group III metal, such as Al, Ga, or In. The hole transporting moiety may be a hole transporting amine moiety. One example of such a hole transporting amine moiety is a triarylamine derivatized phenoxide. The compound may therefore have, for example, two 2-methyl-8-quinolinolato ligands coordinated with Al as electron transporting moieties and one triarylamine derivatized phenoxide as a hole transporting moiety.
The charge carrier layer that includes a compound having molecules having at least one electron transporting moiety and at least one hole transporting moiety may be an emissive layer, such as the ETL of a single heterostructure, or the separate emissive layer of a double heterostructure. This compound may also be used as an injection enhancement layer, such as an electron injection enhancement layer, which is disposed between the ETL and cathode of an OLED, and enhances the injection of electrons from the cathode into the ETL, or a hole injection enhancement layer, which is disposed between the HTL and anode of an OLED, and enhances the injection of holes from the anode into the HTL.
The present invention further provides an ETL having a high hole mobility, so that holes can be transported away from the HTL/ETL interface of a single heterostructure to ultimately recombine with electrons throughout a large volume of the ETL. It is believed that this feature enhances the lifetime of OLEDs.
Further objectives and advantages of the present invention will be apparent to those skilled in the art from the detailed description of the disclosed invention. (